Inorganic/organic composite membranes formed by polybenzimidazole, silicotungstic acid and silica with different ratio between them have been prepared and characterized before and after treatment in phosphoric acid in order to evaluate the influence of composition and acid treatment on some main characteristics of the membranes. In particular the proton conductivity, the mechanical stability and the structural characteristics of the membranes were evaluated. Silica behaved as a support on which the heteropolyacid remained blocked in finely dispersed state and as an adsorbent for water, thus determining a beneficial effect on proton conduction. The membrane with 50 wt.% of SiWA-SiO2/PBI, mechanically stable, gave proton conductivity of 1.2 x 10(-3) S cm(-1) at 160 degreesC and 100% relative humidity. After treatment with phosphoric acid the proton conductivity of membranes increased to 2.23 x 10(-3) S cm(-1) under the same test conditions. All the materials prepared had amorphous structure. (C) 2001 Elsevier Science B.V. All rights reserved.